When I use the executor I see in the JMeter log file a lot of entries with a message like this: "Number of samples in transaction : 6, number of failing samples : 1".

What does it mean?

My test script is a series of actions that is executed 10 times (i.e. 10 iterations). What is really frustrating that the same action is successful in some iterations and fails in others. (As far as I could see, typically fails in the first few iterations and successful in the rest; few might mean "all but the last one").

This (the fails and the pattern of failures) happens with all kinds of actions: change sheet, list box random selection from disabled, clear all, etc. (How can a "clear all" fail???)